There are several types of transmission used in vehicles. Transmissions provided with one input shaft driving a first or a second countershaft can have an independent or a dependent layout. In a transmission with an independent layout, all the driven gearwheels upon the countershafts are driven by a driving gearwheel upon the input shaft, which driving gearwheel does not drive any driven gearwheel upon the other counter shaft. The input shaft must accommodate the sum of all driven gearwheels upon the countershafts. Consequently a transmission with an independent layout is often bulky and heavy, and there is thus a need to improve the solution in terms of size and weight.
In independent transmissions where each gear corresponds to a separate gearwheel set, with a gearwheel arranged upon the countershaft and a driving gearwheel arranged upon the input shaft, the ratio of the gearwheel set is easily adapted by changing one of the gearwheels of the set to a different size. The ratio of the gearwheel set determines the outgoing torque of the gear, which may be adapted depending on the engine etc. Hence, the most common transmission solution is flexible in terms of changing the ratio of the individual gears but bulky and heavy.
The transmissions described above are bulky, and other solutions are thus sought. In order to make possible a transmission of less size and weight the transmission may utilize dependent gears. In dependent gears two gearwheels of two countershafts are both interacting with one common driving gearwheel arranged upon the input shaft. Likewise, two output gearwheels may interact with a common final drive gearwheel. Thereby, fewer driving gearwheels are needed and the dependent transmissions may be made compacter than independent transmissions. Commonly in dependent transmissions the choice of gearwheel sets are restricted by the geometry of the driving gearwheel of the input shaft, the gearwheels of the countershafts and the output gearwheel of the output shaft as these are all dependent on each other. In order to change the ratio of one gearwheel set, comprising a first gearwheel arranged on the first countershaft and a driving gearwheel on the input shaft, the second gearwheel on the second countershaft which is interacting with the same driving gearwheel must consequently be adapted to the change as well. Therefore, dependent transmissions are less flexible than an independent transmission.